This section is intended to provide a background to the various embodiments of the technology that are described in this disclosure. Therefore, unless otherwise indicated herein, what is described in this section should not be interpreted to be prior art by its mere inclusion in this section.
Radio communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such radio communication networks support communications for multiple user equipments (UEs) by sharing the available network resources. One example of such a network is the Universal Mobile Telecommunications System (UMTS), a third generation (3G) technology standardized by the 3rd Generation Partnership Project (3GPP). UMTS includes a definition for a Radio Access Network (RAN), referred to as UMTS Terrestrial Radio Access Network (UTRAN). The UMTS, which is the successor to Global System for Mobile Communications (GSM) technologies, supports various air interface standards, such as Wideband-Code Division Multiple Access (W-CDMA), Time Division-Code Division Multiple Access (TD-CDMA), and Time Division-Synchronous Code Division Multiple Access (TD-SCDMA). The UMTS also supports enhanced 3G data communications protocols, such as High Speed Packet Access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications. For example, third-generation UMTS based on W-CDMA has been deployed in many places of the world. To ensure that this system remains competitive in the future, 3GPP began a project to define the long-term evolution of UMTS cellular technology. The specifications related to this effort are formally known as Evolved UMTS Terrestrial Radio Access (E-UTRA) and Evolved UMTS Terrestrial Radio Access Network (E-UTRAN), but are more commonly referred to by the name Long Term Evolution (LTE). More detailed descriptions of radio communication networks and systems can be found in literature, such as in Technical Specifications published by, e.g., the 3GPP.
Classical Versus Shared Radio Cell Deployments
In the following, the term point is used to mean a point having transmission and/or reception capabilities. As used herein, this term may interchangeably be referred to as “transmission point”, “reception point”, “transmission/reception point” or “node”. To this end, it should also be appreciated that the term point may include devices such as radio network nodes (e.g. evolved NodeB (eNB), a Radio Network Controller (RNC), etc)) and radio units (e.g. Remote Radio Units (RRUs)). As is known among persons skilled in the art, radio network nodes generally differ from RRUs in that the radio network nodes have comparatively more controlling functionality. For example, radio network nodes typically include scheduler functionality, etc., whereas RRUs typically don't. Therefore, RRUs are typically consuming comparatively less computational power than radio network nodes. Sometimes, radio network nodes may therefore be referred to as high power points or high power nodes (HPN) whereas RRUs may be referred to as low power points or low power nodes (LPN). In some cell deployments, LPNs are referred to as pico points and HPNs are referred to as macro points. Thus, macro points are points having comparatively higher power than the pico points.
The classical way of deploying a network is to let different transmission/reception points form separate cells. That is, the signals transmitted from or received at a point is associated with a cell-id (e.g. a Physical Cell Identity (PCI)) that is different from the cell-id employed for other nearby points. Conventionally, each point transmits its own unique signals for broadcast (e.g., PBCH (Physical Broadcast Channel)) and sync channels (e.g., PSS (primary synchronization signal), SSS (secondary synchronization signal)). The classical way of utilizing one cell-id per point is depicted in FIG. 1 for a heterogeneous deployment where a number of LPNs are placed within the coverage area of a HPN. Note that similar principles also apply to classical macro-cellular deployments where all points have similar output power and perhaps are placed in a more regular fashion compared with the case of a heterogeneous deployment.
A recent alternative to the classical cell deployment is to let all the UEs within the geographical area outlined by the coverage of the HPN be served with signals associated with the same cell-id (e.g. the same Physical Cell Identity (PCI)). In other words, from a UE perspective, the received signals appear coming from a single cell. This is schematically illustrated in FIG. 2. Note that only one HPN is shown, other HPNs would typically use different cell-ids (corresponding to different cells) unless they are co-located at the same site. In the latter case of several co-located HPNs, the same cell-id may be shared across the co-located HPNs and those LPNs that correspond to the union of the coverage areas of the macro points. Sync channels, BCH (Broadcast Channels) and control channels may all be transmitted from the HPN while data can be transmitted to a UE also from LPNs by using shared data transmissions (e.g. a Physical Downlink Shared Channel (PDSCH)) relying on UE specific resources. In FIG. 2, the HPN may e.g. be radio network node such as a eNB. The LPNs may be radio units such as those commonly referred to as Remote Radio Units (RRUs).
The single cell-id approach, or combined radio cell deployment (aka shared radio cell deployment or soft radio cell deployment) can be geared towards situations in which there is fast backhaul communication between the points associated to the same cell. A typical case would be a radio network node serving one or more sectors on a macro level as well as having fast fiber connections to remote radio units (RRUs) playing the role of the other points sharing the same cell-id. Those RRUs could represent LPNs with one or more antennas each. Another example is when all the points have a similar power class with no single point having more significance than the others. The radio network node would then handle the signals from all RRUs in a similar manner.
An advantage of the combined cell approach compared with the classical approach is that the typically involved handover procedure between cells only needs to be invoked on a macro basis. Generally, there is also greater flexibility in coordination and scheduling among the points.
Transmission Modes in Combined Cell Deployment
There exist different transmission modes in a combined radio cell deployment. The different transmission modes can e.g. be divided into:
Single Frequency Network (SFN):                In this mode, all nodes generally transmit the same pilot channel. Also, data and control information are transmitted from all nodes. In this mode, only one UE can be served from all the nodes at any time. Hence, this mode can be said to be useful for coverage improvements. Furthermore, this mode works for legacy UEs. As used in this disclosure the expression “legacy UE” is used to mean a UE that supports 3GPP Rel-5, Rel-6, Rel-7, Rel-8, Rel-9, Rel-10, and/or Rel-11. That is, the expression “legacy UE” refers to pre-release 12 UEs.        
Node Selection with Spatial Re-Use:                In this mode, even though all nodes transmit the same pilot channel, data and control information transmitted from one node may be different from the data and control information transmitted from other nodes. For example, a node may be serving a specific UE, while at the same time different data and control information may be sent to a different UE. Hence, the spatial resources can be reused. This mode thus allows for load balancing gains and, accordingly, the capacity of the combined radio cell can be increased.        
In a combined radio cell deployment it is generally a radio network node (sometimes referred to as “the radio network scheduler element” or “the central controller”) that takes responsibility for collecting operational information, operational data or operational statistics from various measurements that are made throughout the combined radio cell. Typically, but not necessarily, the decision of which LPN node (e.g. RRU) that should transmit to a specific UE is made by this radio network node based on the collected operational information, operational data or operational statistics. The operational information, operational data or operational statistics may be collected (e.g. obtained, acquired, or received) from the various LPNs. Additionally, or alternatively, this operational information, operational data or operational statistics may be collected from the UEs that are present in the combined radio cell.
Pilots for Supporting Spatial Reuse Mode in a Combined Radio Cell
In a combined radio cell deployment it may be beneficial or useful to utilize additional pilots, i.e. pilots in addition to normal or regular pilots. As used herein “normal” or “regular” pilots refer to pilots such as common pilots. On the other hand, probing pilots (which will be further explained herein) is an example of an additional pilot within the context of this disclosure.
Additional pilots, such as the above-mentioned probing pilots, may be advantageous in a combined radio cell deployment for various reasons, for example:
Identifying which Node is the Best Available (or Best Suitable) Node for a Particular UE:                In a combined radio cell deployment, all the nodes transmit the same common pilot (C-PICH) and the UE computes, or otherwise determines, a channel quality indicator (CQI) on the basis of the received common pilots. Hence, the central node does not know where the UE is located or which nodes should transmit data to a particular UE. This may be seen as a similarity with cell selection in co-channel deployment, where the UE compares the pilot strengths of each node and decide which radio cell sector is the best available, or most suitable, for the UE in question. In a combined radio cell deployment, since all nodes have the same primary scrambling code, the UE cannot generally distinguish between individual pilots.        
Data Demodulation:                In a combined radio cell deployment, a UE is receiving pilots (or pilots signals) from all the nodes for CQI for channel sounding (e.g., CQI computation), while data is transmitted from only one or a subset of nodes. Hence, the channel estimation for data demodulation may become erroneous if the UE would use channel estimation from combined P-CPICHs. In order to estimate the channel in a better way, it may be advantageous to utilize additional pilots, e.g. probing pilots.        
Pilot Design Options
The 3GPP is studying various pilot design options. Two example pilot design options are described in R1-132603, “Overview of Spatial Reuse Mode in Combined Cell Deployment for Heterogeneous Networks”, which was presented at the 3GPP RAN1#73 meeting in Fukuoka, Japan, 20-24 May 2013. In brief, the first option (i.e. Option 1) introduces additional pilots in the form of probing pilots which can be transmitted continuously at a relatively low power level, whereas the other option (i.e. Option 2) uses demodulation pilots as probing pilots with a comparatively higher power. The first design option (i.e. Option 1) of R1-132603 will now be briefly described:
Separate Probing and Demodulation Pilots:
FIG. 3 illustrates a signaling diagram of example messages when utilizing separate probing and demodulation pilots. Assume that a combined radio cell deployment comprises four nodes (or transmission points) serving multiple UEs. It should be appreciated that the same procedure is applicable also in scenarios where the number of nodes is less than or more than four. A reference signal which is unique to each node in a combined cell called Fractional CPICH (F-CPICH) is transmitted from each node simultaneously and continuously. The F-CPICH is generally characterized by a spreading code (typically SF=256 (SF is an abbreviation for Spreading Factor)) and a scrambling code which is either the primary scrambling code or a secondary scrambling code of the combined radio cell. The F-CPICH channel power levels may be indicated to the UE during the initial cell set up. In addition to F-CPICH, the primary common pilot (P-CPICH) which is common to all the nodes is continuously transmitted. From these two different pilot signals, the UE can estimate the channel and feed back the channel quality information (CQI) associated with these two pilots e.g. at two time intervals. Note that the CQI estimated with F-CPICH indicates the channel quality corresponding to the specific node, referred to hereafter as CQIF, and the CQI computed using P-CPICH is the channel quality using the combined nodes, referred to hereafter as CQIP. These two CQIs are typically, but not necessarily, time multiplexed and sent on the uplink feedback channel HS-DPCCH. The same HS-DPCCH signal is received by all the nodes. The radio network scheduler element (e.g., a radio network node such as a RNC or eNB) can process the received signal (HS-DPCCH) from all the nodes. From CQIF the radio network scheduler element identifies which node the UE is close to. Hence the radio network scheduler element can inform a node in question to transmit to the UE. The assigned node transmits the demodulation pilot channel (D-CPICH), downlink control channel (HS-SCCH) and the downlink traffic channel (HS-PDSCH) to the respective UE. Similarly, the radio network scheduler element informs the other nodes to transmit to the other UEs. Note that D-CPICH and F-CPICH use different spreading codes and may have different power levels. For example, the power level of F-CPICH may be relatively low and D-CPICH may be relatively high.